I've owned both the L300 and S3100 (not the MkII version), and the S3100 is BY FAR the best sounding speaker of the two. Better bass extension, imaging, and HF smoothness. I can only imagine what the improved crossover and driver in the MkII would do for the sound.
That said, the L300 is more valuable and collectible. I sold my L300's in 2009 for $3800 and paid $2100 for the S3100's.
Actually martin_wu99's tip on the 4430 is a good one. I might have gone for a pair for my living room but already had a pair of 4425's in the family room. Too many butt cheeks for one house hahahaha!